This invention relates to novel mold release and lubricant compositions, and is particularly concerned with mold release compositions for protecting and lubricating the surfaces of molds, especially molds employed in the forming and handling of glass articles, e.g. containers, at high temperatures.
In glass manufacturing, particularly the glass bottle or container industry, a paticularly difficult problem has always been the release of the extremely hot glass, at a temperature of up to 1,200.degree. F and higher, from the blank or mold in the first forming operation. Flowing from the furnace, the molten red-hot glass is immediately cut into "gobs", which slide down chutes and drop into the mold, where a bottle is blown as soon as the various mechanisms of the apparatus close around the gob. The blown glass is immediately seized and turned upside down into a second mold, where it is blow again, to its final shape. The entire operation takes only a few seconds.
The mold release composition of the present invention is designed primarily for use with the first mold or blank. The hot glass is extremely abrasive and tends to abrade and tear up the sides of the mold on which the glass container is initially formed. Without suitable lubication of this machined, usually cast iron mold surface, the hot glass could not be removed from the mold and the contours of the mold surface would become rough and constantly changing, making uniform production impossible.
It is accordingly a requirement that the molding surface of the blank mold not become abraded or pitted so that the molten glass will readily flow over the molding surface, take the shape thereof, and then readily separate therefrom. Since the glass is generally heated in the glass-forming equipment to temperatures which can range from 1,000.degree. F to as high as 1,200.degree. F or more, any mold release compositions must be compatible and effective at these high temperatures, without decomposition.
The conventional method for providing glass mold lubrication in the industry has been to "swab" the inside of the mold or blank with a mixture or dispersion of graphite in a light lubricating oil (petroleum distillate), such as kerosene. Since the mold or blank attains an operating temperature of the order of 1,000.degree. F, the oil immediately burns off, producing obnoxious odors and enormous clouds of smoke, and leaving behind a deposit of graphite plus a carbonaceous residue which provide a solid film of lubricant. The problem of smoke formation cannot be alleviated by cooling the mold or blank, because this would chill the glass below the temperature required for blowing the glass to the desired shape. Further, there is a tendency for hard carbon deposits to build up on the mold surfaces thus producing inferior glassware.
In addition, such conventional mold release compositions must be applied to the mold at frequent intervals, requiring additional labor and expense, and with potential increased danger of injury to operators handling the high temperature molds.
As a result, various solutions to these problems have been proposed.
Thus, for example, in U.S. Pat. No. 3,052,629 there is disclosed a glass mold lubricant comprised of a mineral oil base concentrate containing two particular types of dissimilar nonionic surface active agents, a non-abrasive lubricant solid and a water-soluble corrosion inhibitor. The solid lubricant which is added to the oil base in relatively minor amounts can include various carbon blacks, colloidal graphite, mica, and mixtures of such materials with borax and metal oxides such as lead or titanium oxide.
U.S. Pat. No. 3,915,870 discloses a mold release composition containing tungsten disulfide, admixed with graphite or carbon, sodium silicate and a setting agent such as zinc oxide, calcium chloride, magnesium sulfate, and the like.
However, to applicant's knowledge, none of the prior art mold release compositions have been found completely satisfactory.
It is an object of the present invention to provide an improved mold release composition, particularly effective for use as a mold release agent in glass forming operations at high temperatures. Another object is to provide an improved mold release composition of the above type which provides a pre-formed long lasting film on the critical surfaces of the mold or blank, such film having low friction characteristics and having high adherence or tenacity with respect to the mold surface without sticking to the glass, and which is oxidation resistant at high glass forming temperatures up to approximately 1,200.degree. F, and is tough and durable sufficient to survive the rigorous glass forming operations for an extended period, especially up to 18 hours or more, without replenishing.